Polyethylene terephthalate

History

PET was patented in 1941 by John Rex Whinfield, James Tennant Dickson and their employer the Calico Printers' Association of Manchester, England. E. I. DuPont de Nemours in Delaware, United States, first produced Dacron (PET fiber) in 1950 and used the trademark Mylar (boPET film) in June 1951 and received registration of it in 1952. It is still the best-known name used for polyester film. The current owner of the trademark is DuPont Teijin Films.

In the Soviet Union, PET was independently developed in the laboratories of the Institute of High-Molecular Compounds of the USSR Academy of Sciences in 1949, and its Russian name "Lavsan" is an acronym thereof (лаборатории Института высокомолекулярных соединений Академии наук СССР).

The PET bottle was invented in 1973 by Nathaniel Wyeth and patented by DuPont.

Physical properties

PET in its most stable state is a colorless, semi-crystalline resin. However it is intrinsically slow to crystallize compared to other semicrystalline polymers. Depending on processing conditions it can be formed into either non-crystalline (amorphous) or crystalline articles. Its amenability to drawing in manufacturing makes PET useful in fibre and film applications. It is strong and impact-resistant. PET is hygroscopic.

Transparent products can be produced by rapidly cooling molten polymer below the glass transition temperature (T_g) to form a non-crystalline amorphous solid. Amorphous PET forms by rapidly cooling the melt. This glassy state tends to crystallize upon heating the material above T_g, a process known as cold crystallization. Amorphous PET also crystallizes and becomes opaque when exposed to some solvents, such as chloroform or toluene.

A more crystalline product can be produced by cooling the molten polymer slowly. Rather than forming one large single crystal, this material has a number of spherulites (crystallized areas) each containing many small crystallites (grains). Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them, causing the resulting solid to be translucent. Orientation also renders polymers more transparent. This is why BOPET film and bottles are both crystalline, to a degree, and transparent.

Flavor absorption

PET has an affinity for hydrophobic flavors, and drinks sometimes need to be formulated with a higher flavor dosage, compared to those going into glass, to offset the flavor taken up by the container. When returned for reuse, as in some EU countries, PET's propensity to absorb flavors makes it necessary to conduct a "sniffer test" on some returned bottles to avoid cross-contamination of flavors.

Intrinsic viscosity

Different applications of PET require different degrees of polymerization, which can be obtained by modifying the process conditions. The molecular weight of PET is measured by solution viscosity. Viscosity is highly dependent on molecular parameters such as chain length and molecular weight. Due to the structural complexity of branched polymers, viscosity-based determination of molecular weight is best used with linear polymers. With dilute solutions, an empirical relationship can be derived between the viscosity and the hydrodynamic volume and molecular weight distribution. The preferred method to measure this viscosity is the intrinsic viscosity (IV) of the polymer. Intrinsic viscosity is a dimensionless measurement found by extrapolating the relative viscosity (measured in (dℓ/g)) to zero concentration. Shown below are the IV ranges for common applications:

Copolymers

PET is often copolymerized with other diols or diacids to optimize the properties for particular applications.

PETG

For example, cyclohexanedimethanol (CHDM) can be added to the polymer backbone, replacing some of the ethylene glycol. Since this building block is much larger (six additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. In general, such PET is known as PETG or PET-G (polyethylene terephthalate glycol-modified). It is a clear amorphous thermoplastic that can be injection-molded, sheet-extruded or extruded as filament for 3D printing. PETG can be colored during processing. Replacing all of the ethylene glycol with CHDM gives PCT. PETG is also being researched in bioengineering for material in bone tissue engineering.

Isophthalic acid

Another common modifier is isophthalic acid, replacing some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta-) linkage produces an angle in the chain, which also disturbs crystallinity.

Advantages

Such copolymers are advantageous for certain molding applications, such as thermoforming, which is used for example to make tray or blister packaging from co-PET film, or amorphous PET sheet (A-PET/PETA) or PETG sheet. On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM, diethylene glycol (DEG) or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.

Production

Polyethylene terephthalate is produced largely from purified terephthalic acid (PTA), as well as to a lesser extent from (mono-)ethylene glycol (MEG) and dimethyl terephthalate (DMT). As of 2022, ethylene glycol is made from ethene found in natural gas, while terephthalic acid comes from p-xylene made from crude oil. Typically an antimony or titanium compound is used as a catalyst, a phosphite is added as a stabilizer and a bluing agent such as cobalt salt is added to mask any yellowing.

Processes

Dimethyl terephthalate process

In the dimethyl terephthalate (DMT) process, DMT and excess ethylene glycol (MEG) are transesterified in the melt at 150–200 °C with a basic catalyst. Methanol (CH₃OH) is removed by distillation to drive the reaction forward. Excess MEG is distilled off at higher temperature with the aid of vacuum. The second transesterification step proceeds at 270–280 °C, with continuous distillation of MEG as well.

The reactions can be summarized as follows:

First step

C₆H₄(CO₂CH₃)₂ + 2 HOCH₂CH₂OH → C₆H₄(CO₂CH₂CH₂OH)₂ + 2 CH₃OH

Second step

n C₆H₄(CO₂CH₂CH₂OH)₂ → [(CO)C₆H₄(CO₂CH₂CH₂O)]_n + n HOCH₂CH₂OH

Terephthalic acid process

In the terephthalic acid process, MEG and PTA are esterified directly at moderate pressure (2.7–5.5 bar) and high temperature (220–260 °C). Water is eliminated in the reaction, and it is also continuously removed by distillation:

n C₆H₄(CO₂H)₂ + n HOCH₂CH₂OH → [(CO)C₆H₄(CO₂CH₂CH₂O)]_n + 2n H₂O

Bio-PET

Bio-PET is the bio-based counterpart of PET. Essentially, in Bio-PET, the MEG is manufactured from ethylene derived from sugar cane ethanol. A better process based on oxidation of ethanol has been proposed, and it is also technically possible to make PTA from readily available bio-based furfural.

Bottle processing equipment

There are two basic molding methods for PET bottles, one-step and two-step. In two-step molding, two separate machines are used. The first machine injection molds the preform, which resembles a test tube, with the bottle-cap threads already molded into place. The body of the tube is significantly thicker, as it will be inflated into its final shape in the second step using stretch blow molding.

In the second step, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as robust and unique containers themselves; besides novelty candy, some Red Cross chapters distribute them as part of the Vial of Life program to homeowners to store medical history for emergency responders. The two-step process lends itself to third party production remote from the user site. The preforms can be transported and stored by the thousand in a much smaller space than would finished containers, for the second stage to be carried out on the user site on a 'just in time' basis. In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes, etc. Its greatest merit is the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system.

Degradation

PET is subject to degradation during processing. If the moisture level is too high, hydrolysis will reduce the molecular weight by chain scission, resulting in brittleness. If the residence time and/or melt temperature (temperature at melting) are too high, then thermal degradation or thermooxidative degradation will occur resulting in discoloration and lowered molecular weight, as well as the formation of acetaldehyde, and the formation "gel" or "fish-eye" formations through cross-linking. Mitigation measures include copolymerisation with other monomers like CHDM or isophthalic acid, which lower the melting point and thus the melt temperature of the resin, as well as the addition of polymer stabilisers such as phosphites.

Acetaldehyde

Acetaldehyde, which can form by degradation of PET after mishandling of the material, can cause an off-taste in bottled water. As well as high temperatures (PET decomposes above 300 °C or 570 °F) and long barrel residence times, high pressures and high extruder speeds (which cause shear raising the temperature), can also contribute to the production of acetaldehyde. Photo-oxidation can also cause the formation of acetaldehyde. This conversion proceeds via a Type II Norrish reaction.

When acetaldehyde is produced, some of it remains dissolved in the walls of a container and then diffuses into the product stored inside, altering the taste and aroma. This is not such a problem for non-consumables (such as shampoo), for fruit juices (which already contain acetaldehyde), or for strong-tasting drinks like soft drinks. But for bottled water, low acetaldehyde content is quite important, because if nothing masks the aroma, even extremely low concentrations (10–20 parts per billion in the water) of acetaldehyde can produce an off-taste.

Safety and environmental concerns

Commentary published in Environmental Health Perspectives in April 2010 suggested that PET might yield endocrine disruptors under conditions of common use and recommended research on this topic. Proposed mechanisms include leaching of phthalates as well as leaching of antimony. A 2012 article in Journal of Environmental Monitoring concludes that antimony concentration in deionized water stored in PET bottles stays within EU's acceptable limit even if stored briefly at temperatures up to 60 °C (140 °F), while bottled contents (water or soft drinks) may occasionally exceed the EU limit after less than a year of storage at room temperature.

Antimony

Antimony (Sb) is a metalloid element that is used as a catalyst in the form of compounds such as antimony trioxide (Sb₂O₃) or antimony triacetate in the production of PET. After manufacturing, a detectable amount of antimony can be found on the surface of the product. This residue can be removed with washing. Antimony also remains in the material itself and can, thus, migrate out into food and drinks. Exposing PET to boiling or microwaving can increase the levels of antimony significantly, possibly above US EPA maximum contamination levels.

The drinking water limit assessed by WHO is 20 parts per billion (WHO, 2003), and the drinking water limit in the United States is 6 parts per billion. Although antimony trioxide is of low toxicity when taken orally, its presence is still of concern.

The Swiss Federal Office of Public Health investigated the amount of antimony migration, comparing waters bottled in PET and glass: The antimony concentrations of the water in PET bottles were higher, but still well below the allowed maximum concentration. The Swiss Federal Office of Public Health concluded that small amounts of antimony migrate from the PET into bottled water, but that the health risk of the resulting low concentrations is negligible (1% of the "tolerable daily intake" determined by the WHO). A later (2006) but more widely publicized study found similar amounts of antimony in water in PET bottles. The WHO has published a risk assessment for antimony in drinking water. Fruit juice concentrates (for which no guidelines are established), however, that were produced and bottled in PET in the UK were found to contain up to 44.7 μg/L of antimony, well above the EU limits for tap water of 5 μg/L.

Shed microfibres

Clothing sheds microfibres in use, during washing and machine drying. Plastic litter slowly forms small particles. Microplastics which are present on the bottom of the river or seabed can be ingested by small marine life, thus entering the food chain. As PET has a higher density than water, a significant amount of PET microparticles may be precipitated in sewage treatment plants. PET microfibers generated by apparel wear, washing or machine drying can become airborne, and be dispersed into fields, where they are ingested by livestock or plants and end up in the human food supply. A study published in the journal Science of the Total Environment found PET accounted for 18% of microplastics in human lung tissue samples, and that there were 0.69 ± 0.84 microplastics per gram of lung tissue.

SAPEA have declared that such particles 'do not pose a widespread risk'. PET is known to degrade when exposed to sunlight and oxygen. As of 2016, scarce information exists regarding the life-time of the synthetic polymers in the environment.

Polyester recycling

While most thermoplastics can, in principle, be recycled, PET bottle recycling is more practical than many other plastic applications because of the resin's value and the almost exclusive use of PET for widely used water and carbonated soft drink bottling. PET bottles lend themselves well to recycling (see below). In many countries PET bottles are recycled to a substantial degree, for example about 75% in Switzerland. The term rPET is commonly used to describe the recycled material, though it is also referred to as R-PET or post-consumer PET (POSTC-PET).

The prime uses for recycled PET are polyester fiber, strapping, and non-food containers. Because of the recyclability of PET and the relative abundance of post-consumer waste in the form of bottles, PET is also rapidly gaining market share as a carpet fiber. PET, like many plastics, is also an excellent candidate for thermal disposal (incineration), as it is composed of carbon, hydrogen, and oxygen, with only trace amounts of catalyst elements (but no sulfur). In general, PET can either be chemically recycled into its original raw materials (PTA, DMT, and EG), destroying the polymer structure completely; mechanically recycled into a different form, without destroying the polymer; or recycled in a process that includes transesterification and the addition of other glycols, polyols, or glycerol to form a new polyol. The polyol from the third method can be used in polyurethane (PU foam) production, or epoxy-based products, including paints.

In 2023 a process was announced for using PET as the basis for supercapacitor production. PET, being stoichiometrically carbon and H₂O, can be turned into a form of carbon containing sheets and nanospheres, with a very high surface area. The process involves holding a mixture of PET, water, nitric acid, and ethanol at a high temperature and pressure for eight hours, followed by centrifugation and drying.

Significant investments were announced in 2021 and 2022 for chemical recycling of PET by glycolysis, methanolysis, and enzymatic recycling to recover monomers. Initially these will also use bottles as feedstock but it is expected that fibres will also be recycled this way in future.

PET is also a desirable fuel for waste-to-energy plants, as it has a high calorific value which helps to reduce the use of primary resources for energy generation.

Biodegradation

PET biodegrades by ester hydrolysis. Some hydrolase enzymes depolymerize (break apart) PET. The enzymes are PETase and MHETase, which afford 2-hydroxyethyl terephthalic acid and then ethylene glycol and terephthalic acid. Discovered in 2016, these enzymes and their host organisms have received intense scrutiny as possible routes for recycling PET or at least destroying PET waste. A number of hurdles remain, such as the enzymes' thermal instability and slow rates for crystalline PET.